1. Field of the Invention
The present invention relates to a projection exposure apparatus which is required to provide high precision imaging characteristics, for example, for manufacturing semiconductor integrated circuits and liquid crystal devices and, more particularly, is concerned with maintenance of the imaging performance of a projection optical system.
2. Related Background Art
A projection exposure apparatus (for example, a stepper) projects an image of a pattern formed on a mask or a reticle (hereinafter collectively referred to as the xe2x80x9creticlexe2x80x9d) onto a photo-sensitive substrate (a semiconductor wafer or a glass plate to which a photo resist is applied) for forming an image through a projection optical system. Usually, a circuit pattern comprising a transmitting part with a transmissivity of approximately 100% for an illumination light and a shielding part with a transmissivity of approximately 0% for same is formed a reticle (a glass substrate made of quartz or the like). In addition, it has been proposed to use a phase-shifted reticle which is provided with a phase shifter for deviating a phase of light from a specific portion of the transmitting part of the circuit pattern on the reticle as much as xcfx80 (rad) relative to light from another portion of the transmitting part. The phase-shifted reticle allows transferring of a more precise pattern than in the use of the reticle (hereinafter referred to as the xe2x80x9cordinary reticlexe2x80x9d) which comprises only the above-described shielding part and transmitting part. In other words, the former reticle yields an effect for improving the resolution. Representative phase-shifted reticles are a space frequency modulated type disclosed in the Japanese Patent Publication No. 62-50811, a half tone type disclosed in Japanese Patent Application Laid-open No. 4-162039 (the assignee filed as U.S. Ser. No. 780,249 (Oct. 22, 1991)), a shifter-shielded type disclosed in Japanese Patent Application Laid-open No. 4-165352, and an edge-emphasized type. In the case that the phase-shifted reticle is used, a coherence factor ("sgr" value) of the illumination optical system should be optimized.
There have been made various attempts which would enable the transfer of a high precision pattern by optimization of illuminating conditions or improving the exposing method. For example, U.S. Pat. No. 4,931,830 has disclosed a method for improving the resolution and the depth of focus by selecting a combination of an optimum numerical aperture ("sgr" value) of the illumination optical system and an optimum numerical aperture (N. A.) of the projection optical system for each specific pattern line width. The Japanese Patent Application Laid-open No. 61-91662 has disclosed a ring zone illuminating method for limiting an illuminating light, which passes through a pattern formation plane of a reticle and a plane in the illumination optical system (hereinafter referred to as the xe2x80x9cpupil planexe2x80x9d of the illumination optical system) in relation to the Fourier transformation, to a ring zone area. In addition, as the assignee has filed as U.S. Ser. No. 791,138 (Nov. 13, 1991), there has been proposed a deformed light source method for limiting the illuminating light, which passes through the pupil plane of the illumination optical system, to a plurality of local areas which are eccentrically deviated from the optical axis. The deformed light source method is described in detail, for example, in SPIE Optical/Laser Microlithography V Vol. 1674-1992. In any of the above described methods, however, the method itself is not effective for all reticle patterns, that is, the line width and the shape, and it is necessary to select optimum method and conditions of illumination for each reticle or pattern. Therefore, the projection exposure apparatus requires a construction which allows variation of the illuminating conditions in the illumination optical system.
In the projection exposure apparatus, it has increasingly been demanded in recent years to maintain the imaging characteristics (magnification of projection, focusing position, etc.) of the projection optical system at fixed values in high accuracies. Therefore various methods for compensating the imaging characteristics have been proposed and actually applied. For example, a method for compensating variations of the imaging characteristics due to absorption the exposure light of the projection optical system has been disclosed, for example, in U.S. Pat. No. 4,666,273. In this method, a quantity of energy (quantity of heat) which has been accumulated in the projection optical system along with incidence of the exposure light into the projection optical system is calculated in sequence, variations of the imaging characteristics in accordance with the quantity of accumulated energy are evaluated and the imaging characteristics are finely adjusted by the specified compensating mechanism. This compensating mechanism is such that a space between two of a plurality of lens elements is tightly sealed and the pressure in this tightly sealed space is adjusted.
Such high resolution technology as described above is intended to maximally utilize the effective diameter of the projection optical system. However, on the pupil plane of the projection optical system, there is a large difference of light intensity distribution between the ordinary reticle and the phase-shifted reticle. In addition, the light intensity distribution on the pupil plane of the projection optical system largely differs with the illuminating method for the reticle, that is, ordinary illumination, ring zone illumination or deformed light source illumination. Though the projection optical system is highly compensated for aberration, it can be concluded that such compensation is incomplete for the reasons of restrictions in design and manufacture. Consequently, if the light intensity distribution differs on the pupil plane of the projection optical system, that is, if the light path for the illuminating light which passes through inside the projection optical system differs, the illuminating light is affected by a different aberration. An amount of the is effect cannot be negligible for those patterns which have lately tended to be increasingly finer. For example, the focusing position differs with the light intensity distribution of the pupil plane due to a spherical aberration of the projuction optical system. It is known that the magnification of projection and the distortion of image are similarly affected by coma aberration. Particularly, if a change of temperature distrubution occurs in the projuction optical system upon a temperature rise due to absorption of the exposure light by the projection optical system itself, the above described aberration becomes larger to result in a serious problem.
For solving the above problem, it is considered to adapt the projection optical system having compensating means for the imaging characteristics so that the amount of compensation by the compensating means is varied in accordance with the type of reticle and the illuminating conditions of the illumination optical system and the imaging characteristics are compensated to an optimum state in accordance with the light intensity distribution on the pupil plane of the projection optical system. For example, Japanese Patent Application Laid-open No. 62-229838 (filed as U.S. Ser. No. 945,648 (Dec. 23, 1986) by the assignee) has disclosed a method for compensating the imaging characteristics in accordance with the light intensity distribution on the pupil plane of the projection optical system when the numerical aperture, that is, the "sgr" value of the illumination optical system, is changed. In this case, the illuminating conditions of the illumination optical system are primarily determined for one exposure. In other words, the illuminating conditions are not changed for the same reticle and the reticle is illuminated under the same illuminating conditions.
However, various types of patterns having different forming conditions may be formed on the reticle. For example, the normal pattern comprising the transmitting part and the shielding part and the phase-shifted pattern which is partly covered with a phase shifter may be provided together on the same reticle. In exposure of a reticle on which a plurality of patterns are inclusively provided (hereinafter referred to as the xe2x80x9cmixed reticlexe2x80x9d), the light intensity distribution on the pupil plane of the projection optical system can be varied for each pattern. Therefore there has been a problem that, the imaging characteristics of the projection optical system are compensated with an emphasis only on one type of pattern on the reticle, whereby the imaging characteristics of the remaining pattern cannot be compensated and a satisfactory pattern image cannot be obtained on the overall reticle.
The mixed reticle is not limited to a combination of the normal pattern and the phase-shifted pattern. For example, in the case of a reticle composed of only the normal type pattern, the preciseness (line width and pitch) may differ from one pattern to another. A problem as described above may occur with such a mixed reticle. This is because an angle of a diffracted light from the pattern differs depending upon the preciseness of the pattern, that is, the light intensity distribution on the pupil plane of the projection optical system differs with each pattern.
An object of the present invention is to provide a projection exposure apparatus capable of projecting all patterns under satisfactory imaging characteristics even though there are a plurality of different patterns for which forming conditions are different on the reticle.
Therefore, for a projection exposure apparatus which has an illumination optical system for irradiating an illuminating light onto a mask in an approximately uniform intensity distribution, a projection optical system for projecting a mask pattern image onto a photosensitive substrate, and an imaging characteristics compensating means for compensating the imaging characteristics of the projection optical system, the present invention provides input means for entering information regarding respective pattern forming conditions (for example, a type of reticle pattern, preciseness, illuminating conditions, process factors of the photosensitive substrate, etc.) of a plurality of pattern areas on the mask and calculating means for calculating an amount of compensation, by the imaging characteristics compensating means, common to each of a plurality of pattern areas in accordance with the information entered. Exposure control means for interrupting an exposing operation when an amount of compensation common to each of a plurality of pattern areas is not determined may also be provided.
In the present invention, when there are a plurality of patterns (areas) in the specified area on the mask which should be transferred onto a photosensitive substrate only by one exposure, the imaging characteristics for each pattern are to be obtained according to the forming conditions for each of a plurality of patterns and a satisfactory amount of compensation common to each of a plurality of patterns is calculated (determined) from the imaging characteristics. Therefore, the imaging characteristics of the projection optical system can be appropriately compensated for all of a plurality of patterns on the mask and all patterns can be projected under satisfactory imaging characteristics. If a difference in the imaging characteristics with respect to all of a plurality of patterns becomes larger along with an environmental change, absorption of exposure light, etc., and there is no compensation amount common to all patterns, the exposure operation is stopped. Therefore manufacturing of defective products is prevented and the yield rate is improved.